Earlier studies (Spiegelman and Whiteley, 1974) showed that RNA polymerase isolated from Bacillus subtilis infected with phage SP82 has a different subunit structure and a different specificity in vitro than the polymerase from uninfected cells. Several modified polymerases containing host core subunits and different complements of phage-specified small polypeptides have now been purified from SP82-infected B. subtilis. The transcriptional specificity of these forms have been analyzed by competition-hybridization using isolated strands of SP82 DNA and by translation of the RNAs synthesized in vitro.